The present invention is directed generally to a process for making acetic acid from carbon monoxide and methanol, and more particularly to a bimodally operable plant wherein the carbon monoxide is obtained from syngas, or synthesis gas, made by reforming a hydrocarbon and synthesizing the methanol from the synthesis gas in a first mode of operation, or by reforming a lower alkanol and importing methanol for reaction with the carbon monoxide to form the acetic acid in a second mode of operation.
The manufacture of acetic acid from carbon monoxide and methanol using a carbonylation catalyst is well known in the art. Representative references disclosing this and similar processes include U.S. Pat. No. 1,961,736 to Carlin et al (Tennessee Products); U.S. Pat. No. 3,769,329 to Paulik et al (Monsanto); U.S. Pat. No. 5,155,261 to Marston et al (Reilly Industries); U.S. Pat. No. 5,672,743 to Garland et al (BP Chemicals); U.S. Pat. No. 5,728,871 to Joensen et al (Haldor Topsoe); U.S. Pat. No. 5,773,642 289 to Denis et al (Acetex Chimie); U.S. Pat. No. 5,817,869 to Hinnenkamp et al (Quantum Chemical Corporation); U.S. Pat. No. 5,877,347 and U.S. Pat. No. 5,877,348 to Ditzel et al (BP Chemicals); U.S. Pat. No. 5,883,289 to Denis et al (Acetex Chimie); and U.S. Pat. No. 5,883,295 to Sunley et al (BP Chemicals), each of which is hereby incorporated herein by reference.
The primary raw materials for acetic acid manufacture are, of course, carbon monoxide and methanol. In the typical acetic acid plant, methanol is imported and carbon monoxide, because of difficulties associated with the transport and storage thereof, is generated in situ, usually by reforming natural gas or another hydrocarbon with steam and/or carbon dioxide. A significant expense for new acetic acid production capacity is the capital cost of the equipment necessary for the carbon monoxide generation. It would be extremely desirable if this capital cost could be largely eliminated or significantly reduced.
Market conditions, from time to time in various localities, can result in relatively low methanol prices (an oversupply) and/or high natural gas prices (a shortage) that can make methanol manufacture unprofitable. Operators of existing methanol manufacturing facilities can be faced with the decision of whether or not to continue the unprofitable manufacture of methanol in the hope that product prices will eventually rebound and/or raw material prices will drop to profitable levels. The present invention addresses a way of modifying an existing unprofitable methanol plant to make it more profitable when methanol prices are low and/or natural gas prices are high. The present invention also addresses a way of building a new plant with two modes of operationxe2x80x94one with a hydrocarbon feed and the other with an imported methanol feed.
As far as applicant is aware, there is no disclosure in the prior art for modifying existing methanol plants, including methanol/ammonia plants, to supply stoichiometric methanol and CO for manufacturing acetic acid, for example, that can be a more valuable product than methanol. Further, as far as applicant is aware, there is no disclosure in the prior art for modifying existing methanol plants, particularly the steam reformers thereof to reform either a hydrocarbon or a lower alkanol, e.g. methanol, using a hydrocarbon reforming catalyst with the optional presence of carbon dioxide, steam or both.
The present invention involves the discovery that the large capital costs associated with CO generation for a new acetic acid plant can be significantly reduced or largely eliminated by converting an existing methanol or methanol/ammonia plant to make acetic acid. The present invention is equally applicable to a new plant wherein the syngas producing portion of the plant accepts either a hydrocarbon feed, e.g., natural gas, or a lower alkanol feed, e.g., a methanol feed. The steam reformer is built or modified to accept either a natural gas feed or an imported methanol feed and to optionally have a carbon dioxide input, a steam input or both. The reformation takes place in the presence of a hydrocarbon reformation catalyst. Further, all or part of the syngas can be diverted from the methanol synthesis loop and supplied instead to a separator unit to recover CO2, CO and hydrogen, which are advantageously used in various novel ways to produce acetic acid. When the steam reformer is operated with a lower alkanol feed, the methanol synthesis loop is shut down and isolated from the rest of the plant. In this case, all of the synthesis gas will be diverted from the methanol synthesis loop to the separation unit. The recovered CO2 can be supplied to the reformer to enhance CO production, or to the methanol synthesis loop to make methanol. The recovered CO is usually supplied to the acetic acid reactor with the methanol to make the acetic acid. When a lower alkanol feed, e.g., methanol feed, is used for the reformer, methanol from an imported source is also supplied to the acetic acid reactor. The recovered hydrogen can be supplied to the methanol synthesis loop (when in use) for methanol production, used for the manufacture of ammonia or other products, burned as a fuel, or exported, since the hydrogen is normally produced in excess of the requirements for methanol synthesis in the present invention.
The carbon dioxide can be fed into a steam reformer to which (1) natural gas or methanol and (2) optionally steam (water) are fed. Syngas is formed in the reformer wherein both (1) the natural gas or methanol and (2) the carbon dioxide are reformed to produce syngas with a large proportion of carbon monoxide relative to reforming without added carbon dioxide. Alternatively or additionally, the CO2 can be supplied to the methanol synthesis loop (when in operation), with additional CO from the synthesis gas and/or additional imported CO2, for catalytic reaction with hydrogen to make methanol.
In the mode when the methanol synthesis loop is in operation, natural gas is preferably used as the hydrocarbon feed to the steam reformer. The syngas can be split into a first part and a second part. The first syngas part is converted to methanol in a conventional methanol synthesis loop that is operated at about half of the design capacity of the original plant since less syngas is supplied to it. The second syngas part can be processed to separate out carbon dioxide and carbon monoxide, and the separated carbon dioxide can be fed back into the feed to the reformer to enhance carbon monoxide formation, and/or fed to the methanol synthesis loop to make methanol. The separated carbon monoxide can then be reacted with the methanol to produce acetic acid or an acetic acid precursor by a conventional process.
In the mode wherein the methanol synthesis loop is shut down and isolated from the rest of the plant, imported lower alkanol, e.g., methanol, is used as a feed to the steam reformer and imported methanol is used as a feed to the acetic acid reactor. The syngas is processed to separate out carbon dioxide and carbon monoxide, and the separated carbon dioxide can be fed back into the feed to the reformer to enhance carbon monoxide formation. The separated carbon monoxide can then be reacted with the imported methanol to produce acetic acid or an acetic acid precursor by a conventional process.
In the mode wherein natural gas is used as a feed to the steam reformer, the method comprises the steps of: (a) diverting a portion of the syngas stream from at least one steam reformer to a separation unit; (b) operating the methanol synthesis loop with a feed comprising the remaining syngas stream to produce less methanol than the original methanol plant; (c) operating the separation unit to separate the diverted syngas into at least a carbon monoxide-rich stream and a hydrogen-rich stream, preferably wherein the quantity of hydrogen in the hydrogen-rich stream is greater than any net hydrogen production of the original methanol plant; and (d) reacting the carbon monoxide-rich stream from the separation unit with the methanol from the methanol synthesis loop to form the product, wherein the diversion of the syngas stream is balanced for the approximately stoichiometric production of the methanol from the methanol synthesis loop and the carbon monoxide-rich stream from the separation unit for conversion to the product.
In the mode wherein a lower alkanol, preferably methanol, is used as feed to the steam reformer, the method comprises the steps of: (a) feeding the syngas stream from at least one steam reformer to a separation unit; (b) isolating the methanol synthesis loop from the remainder of the plant; (c) operating the separation unit to separate the syngas into at least a carbon monoxide-rich stream and a hydrogen-rich stream; and (d) reacting the carbon monoxide-rich stream from the separation unit with the methanol from the imported source.
Preferably, the at least one steam reformer is built or modified to increase carbon monoxide production in the syngas stream. The steam reformer contains a hydrocarbon reformation catalyst and is used to reform a hydrocarbon, e.g., natural gas, or a lower alkanol (C1-C3 alcohol), e.g., methanol, to syngas. Alternatively, the steam reformer may utilize a methanol reformation catalyst to generate syngas when the plant is operating in the second mode with a methanol feed. The steam reformer is preferably modified to operate at a higher temperature. The syngas stream preferably comprises carbon dioxide, and the separation unit produces a carbon dioxide-rich stream that is preferably recycled to the at least one reformer to increase the carbon monoxide production.
The reaction step can include the direct catalytic reaction of methanol and carbon monoxide to form acetic acid as in the Mosanto-BP process, for example, or alternatively can comprise the intermediate formation of methyl formate and isomerization of the methyl formate to acetic acid, the intermediate reaction of a s mole of CO and two moles of methyl alcohol to form methyl acetate and hydrolysis of the methyl acetate to acetic acid and methanol, or the carbonylation of the methyl acetate to form acetic anhydride.
Separated hydrogen, which is generally produced in excess beyond that required for methanol synthesis in the present process, can also be reacted with nitrogen, in a conventional manner, to produce ammonia. Also, a portion of acetic acid that is produced can be reacted in a conventional manner with oxygen and ethylene to form vinyl acetate monomer. The nitrogen for the ammonia process (especially for any added ammonia capacity in a retrofit of an original methanol plant comprising an ammonia synthesis loop) and the oxygen for the vinyl acetate monomer process, can be obtained from a conventional air separation unit.
Broadly, the present invention provides, in one aspect, a method for converting an original methanol plant to a converted plant for manufacturing a product from carbon monoxide and methanol selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. The original methanol plant comprises at least one steam reformer for converting a hydrocarbon to a syngas stream containing hydrogen, carbon monoxide and carbon dioxide, and a methanol synthesis loop for converting hydrogen and carbon monoxide from the syngas stream to methanol. The method comprises the steps of: (1) providing the original methanol plant; (2) providing for selectively supplying a gaseous feed to the at least one steam reformer, wherein in a first mode the gaseous feed is a hydrocarbon and in a second mode the gaseous feed is a vaporized lower alkanol; (3) installing a vaporizer for vaporizing a lower alkanol from an imported source into the vaporized lower alkanol; (4) loading the at least one steam reformer with a hydrocarbon reformation catalyst for syngas generation; (5) installing a separation unit for separating all or part of the syngas stream into respective streams rich in carbon dioxide, carbon monoxide and hydrogen; (6) providing for diverting all or part of the syngas stream originally fed to the methanol synthesis loop to the separation unit; (7) providing for supplying at least a portion of the carbon dioxide-rich stream to the at least one steam reformer, to the methanol synthesis loop, or to a combination thereof; (8) installing isolation valves for isolating the methanol synthesis loop from the remainder of the converted plant when operated in the second mode; (9) installing a reactor for reacting carbon-monoxide and methanol to form the product; (10) providing for supplying at least a portion of the carbon monoxide-rich stream from the separation unit to the reactor; and (11) providing for supplying methanol to the reactor in the first mode from the methanol synthesis loop and in the second mode from an imported source.
The method for operating this converted plant comprises the steps of: (1) selecting between the first mode and the second mode of operation; and (2) operating the converted plant in the selected mode. The first mode of operation has at least the following steps (1) feeding the hydrocarbon to the at least one steam reformer, (2) operating the at least one steam reformer to generate syngas, (3) separating at least a portion of the syngas stream in the separation unit into respective streams rich in carbon dioxide, carbon monoxide and hydrogen, (4) operating the methanol synthesis loop with a feed comprising (a) carbon dioxide and (b) hydrogen, and (5) reacting at least a portion of the carbon monoxide-rich stream from the separation unit with methanol from the methanol synthesis loop to form the product. The second mode of operation has at least the following steps (1) vaporizing the lower alkanol, preferably methanol, (2) feeding the vaporized lower alkanol to the at least one steam reformer, (3) operating the at least one steam reformer to generate syngas, (4) separating all or part of the syngas stream in the separation unit into respective streams rich in carbon dioxide, carbon monoxide and hydrogen, (5) isolating the methanol synthesis loop from the remainder of the converted plant, and (6) reacting at least a portion of the carbon monoxide-rich stream from the separation unit with methanol from an imported source to form the product. Preferably, the product is acetic acid.
When the first mode is selected, the feed to the methanol synthesis loop can include imported carbon dioxide and/or a portion of the synthesis gas. Preferably, essentially all of the syngas stream is supplied to the separation step. The hydrogen supplied to the methanol synthesis loop is preferably provided by supplying at least a portion of the hydrogen-rich stream to the methanol synthesis loop. The amount of the hydrogen-rich stream is generally in excess of the stoichiometric hydrogen required by the methanol synthesis loop. Preferably, essentially all of the carbon dioxide-rich stream is supplied to the synthesis loop.
In either mode, essentially all of the carbon monoxide-rich stream is preferably supplied to the reaction step. The at least one steam reformer preferably has a second feed comprising a carbon dioxide-rich stream. This may be an imported stream or recycled from the separation unit. The carbon dioxide is converted to carbon monoxide in the reformer. The carbon dioxide-rich stream may be a mixed CO/carbon dioxide stream, for example, in a 1:2 to 2:1 molar ratio.
An imported carbon dioxide-rich stream can be supplied to the methanol synthesis loop (only in the first mode) or to the separation unit, but as noted above is preferably supplied to the reformer for conversion of the carbon dioxide to CO. In addition or alternatively, steam is fed to the at least one steam reformer.
In a preferred embodiment wherein the first mode is selected, the method for operating the modified or retrofitted plant comprises (1) supplying a major portion of the syngas stream to the separation unit for separating the syngas stream into respective streams rich in carbon dioxide, carbon monoxide and hydrogen, (2) operating the methanol synthesis loop with a feed comprising the carbon dioxide-rich stream from the separation unit, a minor portion of the syngas stream, and an additional source of carbon dioxide to produce a methanol stream, and (3) reacting the carbon monoxide-rich stream from the separation unit with the methanol stream from the methanol synthesis loop to form the product.
In another preferred embodiment wherein the first mode is selected, the method for operating the converted plant comprises (1) supplying the syngas stream to a separation unit for separating the syngas stream into respective streams rich in carbon dioxide, carbon monoxide and hydrogen, (2) operating the methanol synthesis loop with a feed comprising the carbon-dioxide-rich stream from the separation unit, a portion of the hydrogen-rich stream from the separation unit, a minor portion of the syngas stream, and carbon dioxide from an additional source, to produce a methanol stream, and (3) reacting the carbon monoxide-rich stream from the separation unit with the methanol stream from the methanol synthesis loop in essentially stoichiometric proportions to form the product.
In another aspect, the present invention provides a process for making hydrogen and a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. The process comprising the steps of: (1) reforming a hydrocarbon in a first mode or a lower alkanol in a second mode with steam using a hydrocarbon reformation catalyst to form a syngas containing hydrogen, carbon monoxide, and carbon dioxide; (2) recovering heat from the syngas to form a cooled syngas stream; (3) compressing the cooled syngas stream to a separation pressure; (4) separating at least a portion of the compressed syngas in a separation unit into a carbon dioxide-rich stream, a carbon monoxide-rich and a hydrogen-rich stream; (5) providing a methanol stream, wherein in the first mode the methanol stream is provided by operating a methanol synthesis loop to react hydrogen with carbon dioxide to form methanol, and in the second mode the methanol stream is provided from an imported source and the methanol synthesis loop is isolated from the remainder of the process; and (6) reacting the carbon monoxide-rich stream from the separation unit with the methanol stream in approximately stoichiometric proportions to form a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof.
In one embodiment, wherein the first mode is selected, the sources of the hydrogen and carbon dioxide to the methanol synthesis loop are a first portion of the hydrogen from the separation unit and the carbon dioxide from the separation unit. Additional carbon dioxide from another source can be fed to the methanol synthesis loop.
With either mode selected, the reforming step is preferably conducted in the presence of carbon dioxide and the syngas produced by the reforming step has a molar R ratio ((H2xe2x80x94CO2)/(CO+CO2)) from about 2.0 to about 2.9. Preferably, the carbon dioxide present in the reforming step is obtained by recycling the carbon dioxide-rich stream to the reforming step.
With the process in the first mode, the method may further include the steps of diverting a major portion of the compressed syngas to a separation unit; separating the syngas diverted to the separation unit into a carbon dioxide-rich stream, a carbon monoxide-rich stream and a hydrogen-rich stream; further compressing the remaining minor portion of the syngas to a methanol synthesis pressure higher than the separation pressure; operating a methanol synthesis loop to convert the hydrogen, carbon monoxide and carbon dioxide in the further compressed syngas into a methanol stream; and reacting the carbon monoxide-rich stream from the separation unit with the methanol stream from the methanol synthesis loop to form a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof, wherein the diversion step is balanced to obtain approximately stoichiometric amounts of carbon monoxide and methanol.
The process preferably has a molar ratio of carbon dioxide to hydrocarbon comprising natural gas or methanol in feed to the reforming step from about 0.1 to 0.5. This feed preferably has a ratio of steam to natural gas or methanol from about 2 to 6. The methanol synthesis loop can be operated substantially below a total maximum combined design throughput of all methanol synthesis reactor(s) in the loop.
The process can further comprise the step of reacting the hydrogen in the hydrogen-rich stream with nitrogen in an ammonia synthesis reactor to make ammonia. The process can also comprise the step of separating air into a nitrogen stream and an oxygen stream and supplying the nitrogen stream to the ammonia synthesis reactor.
Regardless of whether the plant is a converted plant or a new plant, where the product comprises acetic acid, the reaction step preferably comprises reacting methanol, methyl formate, or a combination thereof in the presence of a reaction mixture comprising carbon monoxide, water, a solvent and a catalyst system comprising at least one halogenated promoter and at least one compound of rhodium, iridium or a combination thereof. The reaction mixture preferably has a water content up to 20 weight percent. Where the reaction step comprises simple carbonylation, the water content in the reaction mixture is more preferably from about 14 to about 15 weight percent. Where the reaction step comprises low-water carbonylation, the water content in the reaction mixture is more preferably from about 2 to about 8 weight percent. Where the reaction step comprises methyl formate isomerization or a combination of isomerization and methanol carbonylation, the reaction mixture more preferably contains a nonzero quantity of water up to 2 weight percent. The reaction step is preferably continuous.
Alternatively, the reaction step comprises the intermediate formation of methyl formate and isomerization of the methyl formate to acetic acid. Or, the reaction step may comprise the intermediate reaction of one mole of CO and two moles of methanol to form methyl acetate and hydrolysis of the methyl acetate to acetic acid and methanol.
Where the product comprises acetic acid or an acetic acid precursor which is converted to acetic acid, the process can further comprise the step of supplying the oxygen stream from the air separation unit to a vinyl acetate synthesis reactor, along with a portion of the acetic acid from the carbon monoxide-methanol reaction step, and ethylene, to produce a vinyl acetate monomer stream.
In another embodiment, the present invention provides a method for converting an original methanol plant into a converted plant for manufacturing a product from carbon monoxide and methanol selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. The original methanol plant comprises (1) at least one steam reformer for converting a hydrocarbon to a syngas stream containing hydrogen and carbon monoxide, (2) a heat recovery section for cooling the syngas stream, (3) a compression unit for compressing the syngas stream, and (4) a methanol synthesis loop for converting at least a portion of the hydrogen and carbon monoxide in the syngas stream to methanol. The method comprises the steps of: (1) providing the original methanol plant; (2) providing for selectively supplying to the at least one steam reformer in a first mode a hydrocarbon and in a second mode a vaporized lower alkanol; (3) installing a vaporizer for vaporizing a lower alkanol into the vaporized lower alkanol; (4) loading the at least one steam reformer with a hydrocarbon reformation catalyst for syngas generation; (5) installing a separation unit for separating the syngas fed thereto into respective streams rich in carbon dioxide, carbon monoxide and hydrogen; (6) installing isolation valves for isolating the methanol synthesis loop from the remainder of the converted plant when operated in the second mode; (7) modifying the flow of the syngas stream to allow diverting at least a portion of the syngas stream from the at least one reformer as a diverted syngas stream to the separation unit, wherein the separation unit is configured to separate the diverted syngas stream into at least a carbon monoxide-rich stream and a hydrogen-rich stream, preferably wherein the quantity of hydrogen in the hydrogen-rich stream is greater than any net hydrogen production of the original methanol plant; (8) modifying the operation of the methanol synthesis loop when in the first mode by changing the feed thereto to include at least the remaining syngas stream to produce less methanol than the original methanol plant; (9) installing a reactor for reacting the carbon monoxide-rich stream from the separation unit with methanol to form the product, wherein when in the first mode the diversion of the syngas stream is balanced for the approximately stoichiometric production of the methanol from the methanol synthesis loop and the carbon monoxide-rich stream from the separation unit for conversion to the product; (10) providing for supplying at least a portion of the carbon dioxide-rich stream to the at least one steam reformer, to the methanol synthesis loop, or to a combination thereof; (11) providing for supplying at least a portion of the carbon monoxide-rich stream from the separation unit; and (12) providing for selectively supplying methanol to the reactor in the first mode from the methanol synthesis loop and in the second mode from an imported source.
This method may further include the step of modifying the at least one steam reformer to increase carbon monoxide production in the syngas stream. Wherein the syngas stream comprises carbon dioxide and the separation unit produces a carbon dioxide-rich stream, this stream is preferably recycled to the at least one steam reformer to increase the carbon monoxide production. Wherein the syngas stream in the original plant has a molar ratio R ((H2xe2x80x94CO2)/(CO+CO2)) less than about 2.0 or greater than about 2.9, the syngas stream in the retrofitted plant preferably has an R ratio from about 2.0 to about 2.9.
In another aspect, the invention provides a method for retrofitting an original methanol plant into a converted plant for manufacturing a product from carbon monoxide and methanol selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof. The original methanol plant has at least one steam reformer for converting a feed comprising hydrocarbon and steam essentially free of carbon dioxide into a syngas stream containing hydrogen and carbon monoxide, a heat recovery section for cooling the syngas stream, a compression unit for compressing the syngas stream, and a methanol synthesis loop for converting at least a portion of the hydrogen and carbon monoxide in the syngas stream to methanol. The method comprises the steps of: (1) providing the original methanol plant; (2) providing for selectively feeding to the at least one steam reformer a hydrocarbon in a first mode and a vaporized lower alkanol in a second mode; (3) installing a vaporizer for vaporizing the lower alkanol into the vaporized lower alkanol; (4) loading the at least one steam reformer with a hydrocarbon reformation catalyst for syngas generation; (5) installing a separation unit for separating syngas into a carbon dioxide-rich stream, carbon-monoxide-rich stream and a hydrogen-rich stream; (6) providing for diverting at least a portion of the syngas stream originally fed to the methanol synthesis loop to the separation unit; (7) providing for recycling at least a portion of the carbon dioxide-rich stream from the separation unit to the at least one steam reformer to increase the carbon monoxide formation relative to the original methanol plant and increase the molar ratio of carbon monoxide to hydrogen; (8) installing isolation valves for isolating the methanol synthesis loop from the remainder of the converted plant when operated in the second mode; (9) installing a reactor for reacting carbon-monoxide and methanol to form the product; (10) providing for supplying at least a portion of the carbon monoxide-rich stream from the separation unit to the reactor; and (11) providing for selectively supplying methanol to the reactor in the first mode from the methanol synthesis loop and in the second mode from an imported source. Further, when in the first mode, the diversion of the syngas stream is balanced for the approximately stoichiometric production of the methanol from the methanol synthesis loop using the remaining portion of the syngas stream and the carbon monoxide-rich stream from the separation unit for conversion to the product. In the second mode, the methanol synthesis loop is isolated from the remainder of the converted plant.
The modified steam reformer is preferably modified to operate at a higher temperature to enhance the carbon conversion to carbon monoxide. The separation unit can include a solvent absorber and stripper for carbon dioxide recovery, and a cryogenic distillation unit for carbon monoxide and hydrogen recovery.
The compression unit preferably has a three-stage compressor, and the syngas stream diversion preferably occurs between the second and third compression stages. The third compressor stage is preferably modified for operation at a lower throughput than the original methanol plant. Where the methanol synthesis loop of the original methanol plant includes a recycle loop compressor, the recycle loop compressor can also be modified for operation at a lower throughput.
The method can also comprise importing a stream of mixed CO/carbon dioxide, for example in a 1:2 to 2:1 molar ratio. The imported mixed CO/carbon dioxide stream can be supplied to the methanol synthesis loop or to the separation unit, but is preferably supplied to the reformer where the carbon dioxide therein is substantially converted to CO.
The method can further comprise the step of reacting the hydrogen in the hydrogen-rich stream with nitrogen to make ammonia. Where the original methanol plant produces a hydrogen-rich stream comprising a loop purge from the methanol synthesis loop that was reacted with nitrogen to make ammonia, the retrofitted plant can use the hydrogen-rich stream from the separation unit as a primary hydrogen source for the ammonia production. With the additional hydrogen available from the syngas, additional ammonia can be produced in the retrofitted plant relative to the original methanol plant.
The method can further comprise installing a vinyl acetate synthesis reactor for reacting a portion of the acetic acid with ethylene and oxygen to make vinyl acetate monomer. An air separation unit can be installed to make the oxygen for the vinyl acetate monomer unit, and the nitrogen produced from the air separation unit preferably matches the nitrogen required for the additional ammonia production.
In another embodiment, the present invention provides a method for converting an original methanol plant to a converted plant. The original methanol plant has at least (1) at least one steam reformer for converting a hydrocarbon to a syngas stream containing hydrogen, carbon monoxide, and carbon dioxide, and (2) a methanol synthesis loop for converting hydrogen and carbon monoxide from the syngas stream to methanol. The method comprises the steps of: (1) providing the original methanol plant; (2) providing for supplying a gaseous feed to the at least one steam reformer, wherein the gaseous feed is a vaporized lower alkanol; (3) installing a vaporizer for vaporizing a lower alkanol from an imported source into the vaporized lower alkanol; (4) loading the at least one steam reformer with a hydrocarbon reformation catalyst for syngas generation; (5) installing a separation unit for separating all or part of the syngas stream -into respective streams rich in carbon dioxide, carbon monoxide and hydrogen; (6) providing for diverting all of the syngas stream originally fed to the methanol synthesis loop to the separation unit; (7) providing for supplying at least a portion of the carbon dioxide-rich stream to the at least one steam reformer; (8) installing isolation valves for isolating the methanol synthesis loop from the remainder of the converted plant; (9) installing a reactor for reacting carbon-monoxide and methanol to form a product selected from the group consisting of acetic acid, acetic anhydride, methyl formate, methyl acetate and combinations thereof; (10) providing for supplying at least a portion of the carbon monoxide-rich stream from the separation unit to the reactor; and (11) providing for supplying a methanol stream from an imported source.
The present also provides a method for operating the converted plant. The method comprises the steps of: (1) vaporizing the lower alkanol, preferably methanol, (2) feeding the vaporized lower alkanol to the at least one steam reformer, (3) operating the at least one steam reformer to generate syngas, (4) separating all or part of the syngas stream in the separation unit into respective streams rich in carbon dioxide, carbon monoxide and hydrogen, (5) isolating the methanol synthesis loop from the remainder of the converted plant, and (6) reacting at least a portion of the carbon monoxide-rich stream from the separation unit with methanol from an imported source to form the product. The lower alkanol is preferably methanol. The product is preferably acetic acid.